Four-wheel systems have multiple valves that block additional fluid flow when a wheel slows down too quickly, then, if it does not pick up speed, another valve bleeds off fluid pressure, then, once the speed catches up to the other wheels, a third valve opens to apply stored pressurized fluid. That "block, bleed, apply" sequence occurs 15 times per second. Chrysler controls all four wheels independently. GM uses a similar system but they combine the two rear wheels and just use a three-channel system. If either rear wheel skids, braking pressure is reduced to both rear wheels. During braking, the computer finds the wheel that is rotating the slowest, and modulates its brake pressure if its speed drops below a certain percentage of the fastest wheel's speed.
RWAL, (rear-wheel-abs) works differently. When the rear axle slows down too quickly, brake fluid is blocked, and if more reduction is needed, fluid is bled off. There is no apply circuit and no brake fluid stored under pressure. The apply comes from the brake pedal. As a result, as the system pulses to bleed fluid pressure, the brake pedal will gradually sink. The hope is the truck will come to a stop before the pedal hits the floor.
The important point to understand is a skidding tire has no traction. The purpose of four-wheel-abs is to maintain steering control. The purpose of RWAL systems is to prevent the rear of the truck from trying to pass the front due to loss of rear traction while braking. You will not notice the pulsing pedal and buzzing noise with your system. Also, anti-lock systems do not do their thing below a minimum speed. That is commonly around 15 mph. The other big difference is there is no reference to determine if the rear wheels are slowing too much. The computer only watches the rate of speed decrease relative to time. If the skidding occurs over a prolonged period of time, say four or five seconds, that is long enough for the computer to assume the truck has come to a stop, and it stops running the dump valve. That is why you can still observe skidding tires. And remember, there is no abs action on the front wheels, so those can be locked up any time.
RWAL systems are bled as though they were not even there. There is nothing to activate and no valves to open with a scanner.
You have not listed any history or why you are working on this system. The pressure-differential switch in the combination valve turns the red "Brake" warning light on, along with the parking brake pedal switch, and, if used, a low-fluid level switch in the brake fluid reservoir. To know for sure if the pressure-differential switch is tripped, unplug the wire going to it. That switch is extremely frustrating to reset on Ford products. On Chrysler's and GM's it is spring-loaded and will center itself. Sometimes they stick though, then a good hard jab to the brake pedal will get them broken free once the cause for it tripping has been repaired. The combination valve will not affect bleeding or operation of the brake system.
A common cause of failure to be able to bleed at the rear wheels is the truck is on a hoist and the rear axle is hanging down. Most trucks and minivans have a rear height-sensing proportioning valve to limit the increase in brake fluid pressure under hard braking to prevent easy rear-wheel lock-up. Trucks and minivans can have a wide range of loading with passengers and loads, so the proportioning valve cannot be tailored to the specific vehicle like it can with cars. When the truck is lifted with the rear axle hanging down, it looks to the height-sensing proportioning valve like the truck is very lightly loaded, and little brake fluid pressure should go to the rear wheels. The restricted fluid path can make it hard to bleed that circuit.
Sunday, May 7th, 2017 AT 9:53 PM